Personally Im raising preflop to $2. your 15/1 is not calling with a 4.
This flop would of been alot easier to play.

because this guy is very tight and just as passive, I think JJ or an over pair is very possible. Had you seen this type of move from him before with say AA-QQ?
He must be limping with good hands.
Also bluffing here isnt what I would expect from someone willing to play so patiently.